Special weapons and tactic (SWAT) teams or other police and military personnel, as well as firefighters, may use breaching apparatus to gain entry into locked and barricaded buildings, where criminals, terrorists, hostages, drug caches, and dangerous fires and the like are located. The situations that may be encountered are so varied that usually such teams must be equipped with a large variety of equipment for overcoming a broad spectrum of problems and obstacles that must be overcome to gain entry. In addition, it is highly desirable to minimize damage to surrounding structure when using explosives to gain entry.
For many years, explosive cutting and explosive entry have relied upon shaped explosive charges as distinct from bulk charges. The shaped charge principle, developed by Charles Munroe at the end of the 19th century, is based on the characteristics of shock waves produced when a shaped explosive is detonated. When an explosive charge is placed against a layer of material, e.g. steel, concrete or stone, the shock waves may be directed by the shape of the explosive material, so as to form twin convergent shock wave fronts. The convergent wave fronts are refracted at the surface of the target and reflected from the opposite surface, whereby tensile forces are generated along the centre plane between the shock wave fronts. The tensile forces, depending on several factors such as the shape of the explosive charge, usually resembling in cross-section a sloped roof or inverted V-shape, the thickness of the target and, of course, the amount of explosive can produce a relatively clear linear cut as opposed to fracturing in the case of bulk explosive charge.
It is also known to use a stemming material in the art of directional blasting. Stemming material such as water or sand is used to fill a portion of the explosive device. The stemming material receives a part of the explosive forces during detonation while the material to be blasted receives another part of the explosive forces. Thus, the stemming material directs the blast towards the target. However, the stemming material adds to the weight of the device.
U.S. Pat. No. 4,905,601 to Gabriel et al. describes a device for explosive entry or cutting a dense material such as concrete. The device comprises a backing element comprising a layer of a substantially incompressible material such as e.g. water or sand. The layer is V-shaped in cross-section, the V-shape defining two legs and a cavity therebetween. An explosive charge is accommodated in the cavity and secured in place. The explosive charge has in cross-section a V-shape conforming to that of the cavity. The layer of the incompressible material may be self-contained or encased by an envelope having a V-shape, generally corresponding to that of the incompressible material.
Some devices have been developed that attempt to diminish the production of fragments. U.S. Pat. No. 6,817,297 to Greene, et al. describes a device that uses a flexible material, preferably in a mostly square shape, having substantially orthogonal grooves scored into one side. An explosive charge, usually in the form of a sheet of explosives, is cut to fit the side opposite the grooves, in substantially the same shape as the grooves, without extending beyond the periphery of the flexible material. An initiating means is connected proximately centrally to the explosive charge so that upon initiation, the grooves shape the explosive effect so that a plurality of petals cantilevered are formed in the target material, substantially between the ends of the grooves, to define a fragment-free opening in the target material.